FEATURE: Publish read state on group messages. (#7989) [Undo revert] (#8024)

* Reenable: "FEATURE: Publish read state on group messages. (#7989)"

This reverts commit 67f5cc1ce8.

* FIX: Read indicator only appears when the group setting is enabled

app/controllers/admin/groups_controller.rb

@@ -153,7 +153,8 @@ class Admin::GroupsController < Admin::AdminController
       :default_notification_level,
       :membership_request_template,
       :owner_usernames,
-      :usernames
+      :usernames,
+      :publish_read_state
     ]
     custom_fields = Group.editable_group_custom_fields
     permitted << { custom_fields: custom_fields } unless custom_fields.blank?

app/controllers/groups_controller.rb

@@ -552,7 +552,8 @@ class GroupsController < ApplicationController
             :name,
             :grant_trust_level,
             :automatic_membership_email_domains,
-            :automatic_membership_retroactive
+            :automatic_membership_retroactive,
+            :publish_read_state
           ])
 
           custom_fields = Group.editable_group_custom_fields

app/controllers/post_readers_controller.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+class PostReadersController < ApplicationController
+  requires_login
+
+  def index
+    post = Post.includes(topic: %i[allowed_groups]).find(params[:id])
+    read_state = post.topic.allowed_groups.any? { |g| g.publish_read_state? && g.users.include?(current_user) }
+    raise Discourse::InvalidAccess unless read_state
+
+    readers = User
+      .joins(:topic_users)
+      .where('topic_users.topic_id = ? AND COALESCE(topic_users.last_read_post_number, 1) >= ?', post.topic_id, post.post_number)
+      .where.not(id: [current_user.id, post.user_id])
+
+    readers = readers.map do |r|
+      {
+        id: r.id, avatar_template: r.avatar_template,
+        username: r.username,
+        username_lower: r.username_lower
+      }
+    end
+
+    render_json_dump(post_readers: readers)
+  end
+end